The present invention relates to a data communication apparatus connectable to an ISDN network and capable of performing data communication in a plurality of communication modes such as line switching and packet switching, and to a method of data communication between data communication stations having communication-mode changeover function.
In the ISDN network, data communications can be carried out in two communication modes comprised of line switching and packet switching in the same network. This is one of the communication services which have not been realized in a conventional public communication network.
Data communication using the line switching is an effective means where large amounts of data are transmitted and/or received at high speed. In the public communication network, accounts or charges are made for the time required for data communication. On the other hand, data communication using the packet switching is an effective method in a case where low-speed data communications are made and small amounts of data generated or created in a discrete manner are transmitted and/or received. In this data communication, charges are made for the amount of data transmitted.
When the level or density of generation of data varies between the two stations which perform data communications continuously over a long period of time, the execution of data communication in a communication mode suitable for the density of generation of the data is most effective. It is necessary to change over the communication mode from the line switching to the packet switching or vice versa under data communication in order to realize this.
However, when it is desired to perform a highspeed data communication, there has heretofore been used a digital dedicated line of such a type that a fixed rate charged for data communication is paid for each month. It was therefore unnecessary to take into consideration variations in the density of generation of the data. As disclosed in Japanese Patent Publication Laid-Open No. 62(1987)-290242, this disclosure describes a conventional data communication apparatus of a type wherein the amount of information processed in a central processing unit is supervised, and when its amount exceeds 80% of the information-processing amount, which is defined as the threshold value, the data receiving rate is reduced, whereas when it is below 80%, the data receiving rate is raised, without changing the communication mode from the line switching to the packet switching or vice versa. On the other hand, when it is desired to change the communication rate, this has been realized by performing the data transfer, i.e., data transmission/reception in a escape sequence form between communication terminal apparatus.
Further, Japanese Patent Publication Laid-Open No. 61(1986)-158244 discloses a facsimile apparatus capable of performing data communications in the two communication modes described above. However, the disclosure cannot perform the communication-mode changeover under data communication because the communication-mode changeover is simply performed in hardware form in advance of the data communication.
The above-described prior art has a problem that it is used to change the communication rate according to the amount of the generated data corresponding to the processing rate in the CPU and it does not take into consideration a system of such a type that the accounts or charges are made for the time required for the data communication as in the public communication line, thus the more the communication rate being reduced, the more the amount of accounts or charges increasing. In addition, the prior art also has another problem that all the data are needed to be supervised in the data communication apparatus because the communication rate is varied and the data transmission/reception is performed in the escape sequence form, thus increasing processing overhead time.